This invention relates to scanners for devices such as scanning tunneling microscopes and atomic force microscopes and, more particularly, in a scanning device employing a nonlinear piezoelectric scanner having an attached end and a free end to produce a lateral scanning motion of the free end a distance d about a center position by the connection of a changing scan voltage to electrodes carried by the piezoelectric scanner, to the method of making the scanning motion of the free end linear with time comprising the steps of, providing a nonlinear signal with time which is a function of the nonlinearity of the piezoelectric scanner; amplifying the nonlinear signal to provide a corresponding nonlinear driving voltage; and, applying the nonlinear driving voltage to the electrodes of the scanner.
In a scanning probe microscope such as a scanning tunneling microscope (STM) or an atomic force microscope (AFM), a probe is scanned across the surface of a sample to determine properties of the surface such as topography or magnetic field strength so that these properties can be displayed for viewing. Alternately, the sample can be scanned across a fixed probe. Some of these microscopes, i.e., the STM and the AFM, have been constructed with the ability to resolve individual atoms by either scanning the probe or the sample. The scanner which provides the motion is usually a piezoelectric device adapted for moving in all three dimensions, i.e. in the X-Y plane and in the vertical (Z-axis) direction. As can be appreciated, if one is to resolve movement of a probe to the atomic level, the actuating mechanism must be stable and accurately moveable in small increments.
Recently, the three dimensional scanners have been made in the form of a tube whose probe- or sample-carrying end can be made to deflect in the X, Y and Z directions through the application of voltages to various electrodes on the tube. This type of positioner was first disclosed by Myer in U.S. Pat. No. 4,087,715 and was first reported for an STM by Binning and Smith in the Review of Scientific Instruments, Vol. 57, pp. 1688-1689, (1986), a copy of both having been filed in our co-pending application Ser. No. 305,637, filed Feb. 3, 1989 and entitled SCANNER FOR A SCANNING PROBE MICROSCOPE, the teachings of which are incorporated herein by reference. As depicted in simplified form in FIG. 1, the scanner 10 has electrodes on the outside and inside to which voltages are applied to cause the scanning action. The scanner 10 is attached to a structure at 12 and has a free end at 14 to which the probe 16 (or sample) is attached. By applying a voltage to some electrodes, the scanner 10 can be made to elongate and shorten as indicated by the dashed arrows and thereby create motion in the Z-axis. Likewise, by applying a voltage to other electrodes, the scanner 10 can be made to deflect the free end 14 to one side or the other, or both, and thereby create motion in the X- and Y-axes as indicated by the ghosted positions of FIGS. 1 and 2. The unique scanner of our above-referenced co-pending application is shown in FIGS. 3 and 4 wherein it is generally indicated as 10'. It features multiple inner and outer electrodes for scanning. It is shown in FIG. 3 as it would appear when scanning a sample 18 with respect to a fixed probe 16 and in FIG. 4 scanning a probe 16 over a fixed sample 18. The outer electrodes 20 and 22 are disposed on the surface of a tube 24 formed of a piezoelectric material. The electrodes 20 are X and Y electrodes which are disposed adjacent the attachment end 12 at 90.degree. intervals with only a small break between them. The outer electrode 22 is a Z electrode which covers the full 360.degree. of the outer surface adjacent the free end 14. The inner electrode 20' and 22' are in a similar pattern. The inner and outer scan electrodes are all connected to a scan driver 26 which applies the required voltages to the electrodes to create the desired scan pattern of the free end 14. The electrodes 20 and 20' are for scanning (i.e. movement of the probe 16, for example, in the X and Y directions) by bending the tube 24 as depicted by the ghosted positions in FIGS. 1 and 2. It is this X and Y scanning motion that is the primary problem area to which the present invention is directed. As those skilled in the art will readily recognize, scanners other than tubes can be used within the scope and spirit of the present invention even though the primary thrust of the disclosure is directed to piezoelectric tube scanners. For example, one could scan according to the methods of the present invention employing a bimorph bender as described by Gehrtz et al., J Vac Sci Technology, A6, Mar. 1988, p 432.
With acceptance and contemporary usage of such devices, it has become important to make scanning probe microscopes which have large scan ranges (up to several microns) and good mechanical stability. The motion of the piezoelectric scanner is essentially proportional to the electric field in the piezoelectric material, which is equal to the voltage across the material divided by the thickness of the tube. As described in out above-referenced co-pending application, complementary voltages x, -x and y, -y can be applied to the inner and outer scanning electrodes to give larger scan ranges and more symmetry. While such techniques can provide the larger scan range of movement desired, the inherent nature of the piezoelectric material used to form the tubes 24 begins to create problems of its own as the scan distance (i.e. the amount of bending created in the tube 24) is increased.
Scanning is typically accomplished in a so-called "raster" fashion such as that of the electron beam which creates a television picture; that is, the probe 16 (or sample) moves in, for example, the X direction at a high rate and in the perpendicular direction, i.e. the Y direction, at a low rate to trace out a path such as that indicated as 28 in FIG. 5. Data about the height, magnetic field, temperature, etc. of the surface 30 of the sample 18 is then collected as the probe 16 is moved along. In these scanners, the X and Y position of the probe 16 is inferred from the voltages which are applied to the electrodes on the piezoelectric material of the tube 24. In the prior art, these scan voltages (from the scan driver 26) are typically triangle functions in X and Y (vs time) to produce, if the deflection of the scanner is linear with voltage, a raster scan of the probe in both the X and Y directions. Often, DC voltages are also added to the scan electrodes to position the raster scan over differt areas of the sample surface; that is, to select where on the sample the center position of the raster scan will be. The triangle function has the feature that the voltage, and therefore presumably the probe position, changes at a constant rate so that the probe moves at a constant velocity back and forth in X while moving at a lower constant velocity up and down in Y. This constant velocity then allows data taken at constant time intervals (as the probe moves in X and Y) to also be spaced at constant distance intervals. Since computers can conveniently take data at constant time intervals, they could then plot the data in a two-dimensional array representing position, i.e. in an X-Y array. The motion in X and Y will usually consist of small steps because the scan voltages from the scan driver 26 are changed in finite increments, as a computer would do.
As the field of scanning probe microscopes has progressed and larger scans of up to several microns have been produced so that, for example, the properties of manufactured objects such as optical disks and magnetic recording heads can be measured, the inherent properties of the piezoelectric materials employed in the tubes has begun to affect the above-described scanning process adversely. This is because, unfortunately, piezoelectric material, especially that of high sensitivity, is not a linear material; that is, the deflection of the material is not linear with the voltage applied to the electrodes. Also, the material has hysteresis so that reversals in the direction in which the voltage is changing do not produce a proportional reversal in the direction in which the position of the probe changes. Thus, a triangular voltage in time applied to the electrodes on the piezoelectric material in the manner of the prior art as described above does not produce a linear scan in time. This is illustrated in FIG. 6 where the position of a probe on the scanner as a function of the driving voltage for a one dimensional scan is graphed in simplified form. Notice that as the direction of the voltage changes at the ends of the scan, the position does not trace out the same path. This property of piezoelectric materials is well known and is classified as either hysteresis or "creep". For example, it is noted in U.S. Pat. No. 4,689,516 and also illustrated in piezoelectric material catalogs such as that from the Tokin Corporation of San Jose, Calif.
Sensitivity variation with voltage and the hysteresis make it such that the position of the probe is not linear with the voltage applied to the electrodes on the piezoelectric material. The prior art solved this by putting capacitors in series with the electrical leads to the electrodes as shown in a diagram from a Japanese paper by Sie, et al. (a copy of which is filed herewith). The creep of a piezoelectric material over 200 seconds is shown before and after insertion of capacitors whose size is, order of magnitude, equal to the capacitances of the piezoelectric device. Although this approach improves the creep of the piezoelectric material after voltage is applied to the device, much of the applied voltage appears across the added capacitors, reducing the total voltage which can be applied to the piezoelectric device and limits the range of scanning. Accordingly the capacitor solution to the creep problem is self-defeating to the primary objective of obtaining larger scans and is, therefore, an unattractive and unacceptable solution.
Another possible solution to the hysteresis problem is to apply triangular voltage patterns to the electrodes, let the piezoelectric material scan in a non-linear manner, take data spaced evenly in time (but not space) and then correct the positions of the data in the X-Y array to form a linear array. This would require interpolation of all the data points and would also make it difficult to produce realtime linear images since many calculations would be required as the data is being collected; but, of course, the difficulty of this solution depends on the computing power available. For the smaller laboratory on a limited budget for equipment, the computing intensity and, therefore, the computing power that must be available to implement this approach is probably prohibitive. Incidentally, this method is described by Gehrtz et al. in their previously mentioned article.
Wherefore, it is an object of this invention to provide a method for operating conventional prior art piezoelectric scanners to produce a raster scan that is evenly spaced in both time and space.
It is another object of this invention to provide a method for operating conventional prior art piezoelectric scanners to produce a raster scan which automatically adjusts for changes required by different scan distances.
Other objects and benefits of the invention will become apparent from the description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.